Sunday, August 25, 2013

Smell may be the least neurologically complex system of the
sense as the information from the smell sensors in the nose have a direct link
to the processor in the brain. Taste is considered one sense but in fact there
are 5 specific receptors (located on the tongue in areas) for salty, sweet,
tart, sour and what is being called “meat” flavors. These five flavors are
given more variety by the sense of smell which distinguishes variations within
these five flavors of foods. Often, in high school biology class, teachers do
an experiment where they stop the sense of smell and offer a bite of an apple
and of an onion to a student. Without smell the student cannot tell which is
which as the texture is similar and both are sweet. Smell is one of the senses that disappears in
the aging process and older people often lose interest in most foods, except sweets,
because the food becomes bland to them without smell. Smell is an important first indicator of some
dangers as it can detect more remote sensation than taste. To taste something
it must touch the tongue but smells waft through the air to your nose.

The system for
smell develops between 16 and 24 weeks of gestation and is in place and ready
to function by about 24 weeks gestation. The first taste buds appear at 8-9
weeks gestation with most being present by about 16 weeks gestation. Some
refinement continues but most full term babies have a fully developed taste
system at birth.

The system of smell is perhaps the strongest sense in
infants and babies are able to recognize the smell of their mother’s breast
milk from that of another person (the study was done using breast pads) almost
from birth. They also respond to strong smells with avoidant movements from an
early age. I would avoid strong perfumes and air fresheners around very young
infants as these might be unpleasant to their sensitive system. In a study (titled
“Natural odour preferences of newborn infants change over
time.” by [My paper]H Varendi, R H Porter, J Winberg) it was shown that babies preferred
their mother’s unwashed natural breast over the washed and soapy smelling other
breast during the first week to 10 days of life. It has been proposed that smell is the
first sense used in socializing as newborns begin to recognize different people
by their smell, which allows them to respond socially to those they know and
those they do not know from a very early age.

These systems rely on experience after birth to become
completely developed. The brain records tastes and smells so that a child can
recognize what they mean. Infants can register likes and dislikes from the
start. Due to their limited experience, infants are happy with relatively bland
food and do not need salt or sugar added to foods to want them. However, they
recognize variations in their food and may reject food that tastes

“funny”.
Many breast feeding mothers report that their baby is not interested in the
breast after they have eaten a certain type of food. Which foods trigger that
response are different for each baby but all are strong flavored foods. In my
case, it was pickled foods that I had to avoid. This is learned by the baby through
experience with foods and by the mother in watching her baby’s reaction to feeding.

On the other hand, older people who have lost some of their
sense of smell are best able to taste the flavors that are tasted by the
tongue. Sweet is the strongest, being at the front of the tongue, followed by
salt. Older adults will often only express interest in sweet foods and will
often over salt foods in order to experience variation in tastes. Many older
adults will say that food these days doesn’t taste like it used to. For them, it
is probably true.

It is thought that these two systems have some protective
functions as most really bad smells and bitter tastes are not good for people
so your own body can offer some protection against certain poisons. However the
system is not foolproof and some unpleasant tasting things are not harmful
while some harmful things are not unpleasant. It has been noted that during
pregnancy, many women are exceptionally sensitive to smells, which may be a
primitive form of guiding them towards the particular foods they need.

In any case, the sense of smell is one of the least studies
and most underrated the special senses. Below are some links to references on
the sense of smell:

Your child’s sensory motor system is a very important part
of his development. In fact, almost all human activity is based in the sensory
motor system. Basically, we take in information through our senses and then we
react to it, usually with a motor response of some sort. The system is very complex and there is
coordination (it is a pre internet type of web) among all the senses and the
motor systems of the body but I will try to keep it simple here. Something
happens in the environment and the sense organs pick it up. The “sensation” is
transmitted to the brain where it is processed (and coordinated with other
information from other senses) interpreted and a response is formulated. This
information goes to the motor cortex of the brain where instructions to your
body are transmitted and you react. All this takes place in nanoseconds so your
response may seem immediate, certainly much faster than it took to read about.
Let’s use a hot iron as an example. Your finger touches the iron and the pain
and thermal sensors in your finger recognize the sensation. It is sent to the
brain which registers “HOT” and dangerous. The information is sent to the motor
areas of the brain which determines that your muscles in your arm must activate
to move your finger away from the danger. This information is sent to your
finger and you pull your finger away from the iron. Most of us have had this
experience so you know how fast this system works.

In looking at the sensory part of this system we need to
understand what the senses are. Firstly, the system consists of three basic
parts, the sensory receptor, the nerve transmitter and the part of the brain
that processes the sensation. The receptors are the organs that come in contact
with the sensation, the nerve endings throughout the body and especially around
the head that receive the sensation. The
nerves are the transport system, much like telephone cables that transmit the
sensation to the brain. All of the understanding of what was sensed takes place
in the brain. Each sense is processed in its own part of the brain separately,
although there are connections so that this information can be coordinated to
form a whole picture. This interconnection also helps when there is an
impairment in one of the senses, such as blindness, which allows information
from other senses to help “cover” for the impaired sense in gaining a better
picture of the environment.

Most of us have heard of the five senses. Many of us have
heard the term the sixth sense used to refer to unexplained abilities. Well, the truth is that we have more than 5
senses so that expression is going to have to change to the 10th
sense or something like that.

The first 4 senses are special senses, so called because they
have a single sense organ and detect a single type of sensation. These are sight, hearing, smell and
taste. Each of these has a specific
sensory receptor located around the head.
For sight it is the eyes, for hearing it is the ears. Smell and taste
are quite related and are often called the chemosenses.

The remaining senses are called the somatosenses because
they affect the body as a whole. These include touch, which is really several
senses combined under one name, the proprioceptive sense and the vestibular
sense. The first two have sense receptors spread throughout the body while the
vestibular sense uses some of the same structures in the ears that are used for
hearing.

In the next few articles, I will look at the development of
each sense both in terms of the development of the structures and of the sense
itself. Most of the sense organs develop in the absence of stimulation but
processing continues to develop with experience and, in later years , many of
the senses become weaker due to changes in the structures. After looking at
each sense in depth I would like to discuss sensory processing and the area of
sensory processing dysfunction, which is in the process of becoming a diagnosis
as well as being a symptom in several other disorders, including autism.